Optical elements composed of various kinds of optical glass are widely used for optical pickup lenses for discs such as CD, DVD, BED and HD-DVD, and used for image pickup lenses installed in portable telephones. Recently, demand to the optical element composed of such the optical glass is rapidly grown accompanied with the fast spreading of optical-disc recorder/player and portable telephone with camera. Consequently, rising in the production efficiency and lowering in the cost of the optical element is required. Consequently, a press molding method becomes to be widely applied for producing such the optical element. According to such the method, the production process can be reduced compared to usual polishing method so that the optical element can be produced in shorter time and at lower cost.
The press molding method can be classified into a re-heating method and a direct press method. In the re-heating method, a gob preform or a polished perform in a shape which is resembles in the final shape of the product is prepared and the preform is re-heated at a temperature not lower than the yield point At of the glass and pressed to obtain the final shape of the product.
In the direct press method, molten glass is dropped from a glass melting tank through a dropping nozzle onto a mold which is previously heated at a temperature near the glass transition temperature Tg of the molten glass drop, and the molten glass drop is pressed into the shape of the final product before the molten glass drop is cooled and solidified. In this process, repetition of heating and cooling of the mold is not necessary and the final product of the optical element can be directly produced from the molten glass drop. Therefore, the time necessary for once formation of the product is very short. Thus, this method gets attention as a method by which the product efficiency higher than that of the re-heating method can be expected.
As described above, it is necessary to heat the mold to a temperature not lower than the yield temperature At of the glass in the re-heating method or to a temperature near the glass transition temperature Tg of the glass in the direct press method. Consequently, the degradation of the mold is easily accelerated and the life time of the mold is shortened when the yield temperature At or the glass transition temperature Tg of the glass is higher, which results in increase of the cost. Therefore, an optical glass having the yield temperature At or the glass transition temperature Tg as low as possible is preferred for the glass to be used in the press molding method. Particularly, in the case of the direct press method, the production cost can be lowered by the use of the optical glass having low glass transition temperature Tg, 540° C. or less for example, because the optical glass can be formed in the atmosphere and the production equipment can be simplified.
It is preferable that the optical glass does not contain PbO, As2O3, TeO2 and fluoride considering the working environment at the production process and ensuring the safeness of workers.
Hitherto, some kinds of optical glass having high refractive index, high dispersion and low glass transition temperature Tg have been proposed; cf. JP-A Nos. 2005-306733 and 2006-131480 for example.
However, there has been a problem that clouding appears on the optical surface of the produced optical element when the pressing is repeatedly carried out in the direct press method using the optical glasses described in JP-A Nos. 2005-306733 and 2006-131480.
It is supposed that the clouding is caused by that the slight amount of the glass composition adheres onto the mold when the mold is directly contacted to the molten glass drop and the adhering composition re-adheres onto the surface of the next produced optical element. When the pressing process is further repeated, there has been a problem that the surface roughness of the mold is raised by the adhering material and is transferred to the optical element so that the surface roughness of the optical element is raised.
Moreover, there has further been the problem that the clouding also appears on the surface of the optical element by repeating the pressing process in the case of using such the optical glass also in the re-heating method.